Infrared signature control mechanism

ABSTRACT

A means of reflecting and emitting electromagnetic energy in an appropriate wavelength band comprising an arrangement (10) of surfaces (11) which are reflective to energy in that wavelength band and energy emitters (12) having an emission of energy of such intensity that the combined reflection and emission of said surfaces match energy of a background in that wavelength band thereby camouflaging the surfaces. The said emitters (12) comprise strips of material which, upon energizing with an electric current, become heated and radiate energy. The means further comprises at least one radiometer (17) in association with a comparison means to provide an electrical signal which is a function of the difference between the combined reflection and emission and of the background, the electrical signal controlling the energization of the energy emitters (12).

This invention relates to means and method for reflecting and emittingelectro-magnetic energy and although not directly restricted to infraredenergy, the invention is particularly suitable for reflectance andemittance for such energy to a degree which will provide a compositeenergy emission which corresponds to the surrounding environment so thatthe device can be used for camouflage purposes and thereby reduce thedanger of detection by surveillance systems or the danger of a trackingdevice of a missile detecting its target against the background.

BACKGROUND OF THE INVENTION

Surveillance systems and missiles frequently make use of infrareddetectors capable of comparing temperature differentials between objectsand their backgrounds by comparing their emitted infrared energy, insome cases where the differential is as low as 0.1° K. Missile detectorsusually rely largely on the existence of a radiation contrast betweenthe target and the background area, the net radiation from each beingcaused by both reflection and emission from their surfaces. Thedetectors are often operable in the wavelength ranges of from 3 to 5 and8 to 14 micrometers. The wavelength band with which this invention isconcerned extends throughout the infrared range and can also be appliedto the ultraviolet, visible and millimeter wavebands.

It is known that reflectors have been made to reflect for example theenergy from the local environment in the case of an object to becamouflaged so that the detector will fail to "recognise a target", butthat system has limitations and is only partially effective, due todifficulty in selecting a region of the local environment to bereflected which has the same radiance as the background to be matched.

It is therefore an object of this invention to provide means which willminimise the differential of the combination of radiation andreflectance between a potential target and its background therebyconstituting a "camouflage".

BRIEF SUMMARY OF THE INVENTION

In this invention there is provided on an object to be camouflaged anarrangement of surfaces which comprise reflecting surfaces and energyradiating surfaces, and by measuring the background emission and objectemission, control of the energy radiating surfaces can effect a matchbetween the two.

The energy which is received from the sky at high elevation angles,particularly at night is equivalent to that from a black body source ata low temperature, often in the range of 240° K. to 250° K. The energyemanating from a sea surface (for example) is equivalent to that of ablack body in the range of 270° K. to 300° K. A background at seatherefore is likely to have a wavelength emission approximating that ofa black body of temperature between 260° K. and 290° K. depending onaspect which effects the sea surface emissivity and reflectivity.

Energy from a grey-body source having a temperature of T° K. andemissivity of ε is a function of the temperature and emissivityaccording to the formula σεT⁴, where σ is Stefan's constant.

Energy reflected from a surface of emissivity ε is a function of theblack body source of Temperature T₁ which is reflected according to theformula σ(1-ε) T₁ ⁴. If the source being reflected is a grey-body ofemissivity ε₁ this formula becomes σ(1-ε)ε₁ T₁ ⁴.

The wavelength at which the maximum emission occurs is also a functionof source temperature. For a comprehensive understanding of radiated andreflected energy, reference may be made to chapter 1 of the publicationRADIATION THEORY by W. L. Wolfe and George J. Zissis, published by theOffice of Naval Research, Department of the Navy, Washington, D. C.,U.S.A.

The energy difference normally detected by an infrared seeker system atmaximum range is usually that equivalent to a black body temperaturedifference in the range of 1° K. to 5° K. Detectors do not discriminatedifferent wavelengths in the individual wavebands with which thisinvention is concerned.

Therefore by controlling the temperature of the heated areas of acombined reflection/radiation arrangement, the net radiated energy canbe made to match that a background environment in the waveband required.

More specifically, the invention consists of an arrangement of surfaceswhich are reflective to energy in a wavelength band and energy emittershaving an emission of energy of such intensity that their combinedreflection and emission match energy of a background in that wavelengthband.

An embodiment of the invention is described hereunder in some detailwith reference to, and is illustrated in, the accompanying drawings, inwhich:

FIG. 1 is a diagrammatic representation showing:

FIG. 1(a) an arrangement of reflecting surfaces,

FIG. 1(b) the surfaces of (a) drawn to a larger scale, and

FIG. 1(c) an alternative reflecting surface arrangement; and

FIG. 2 is a simplified block diagram of an electrical circuit.

In this embodiment a reflecting surface arrangement 10 (FIGS. 1(a) and1(b)) comprises a plurality of reflecting strips 11 of low emissivityand between these are located a plurality of hot metal radiating strips12 of high emissivity. The reflecting strips 11 reflect the radiationfrom the sky normally equivalent to radiation from a surface oftemperature usually between 253° K. and 240° K. while the radiatingstrips 12 can be heated to produce a temperature higher than the ambienttemperature, usually between 0° and about 325° K.

By means described below the combined energy emissions from thereflecting surfaces and the radiating surfaces can be controlled bycomparator means to match the background emissions.

Background emissions 24 are directed to a radiometer via an aperture 13and an aperture within a chopper disc 14. Emissions from the object 10after reflection from a mirror surface 15 and via aperture 16 arereflected off the chopper surface. Thus as the chopper disc 14 rotates,the background and object emissions are directed to the radiometer 17alternately.

The rotation of the chopper disc 14 is controlled by a motor (not shown)which is controlled by circuit 18 and input 19. The rotation of thechopper disc 14 is also detected by sensor means 20. The sensor meanssignal output is processed by conditioning circuit 21 to provide atrigger control to a switching circuit 22. This processed signal outputis also used by the chopper motor control circuit 18. Output 23 can beused to monitor the conditioning circuit 21 output.

The source switching circuit 22 directs the output of the radiometer 17to the background pulse integrator 25 when the chopper aperture allowsbackground emissions 24 through and directs the output of the radiometer17 to the object pulse integrator 26 when radiation from the object 10(reflecting/emitting surface) is reflected from the chopper into theradiometer.

The pulse integrators 25 and 26 output a voltage level representative ofthe received emissions from the two sources. They feed into adifferential amplifier 27 which is biased to output a voltage levelwhich varies in response to the difference between the receivedemissions. These processes of detection, amplification, integration andcomparison could equally be performed by microprocessor means.

the output of the differential amplifier 27 is fed via wire means to aradiating strip/s driving circuit 30.

This driving circuit controls the current flow through the current loop31 and through the radiating strip elements 12, varying the flow untilthe combined energy reflected and emitted from the arrangement 10, onthe object is the same as that from background 24.

Known infrared detectors have limited spatial discriminationcapabilities and the present maximum resolution allows differences ofsource temperature to be detected at 100 cms apart when viewed from 10kms away. Therefore it is advantageous in application that the geometryof the arrangement allows for surfaces to be sized with less than 100cms effective separation, while maintaining an optimum reflectivesurface angle and radiating strip width, but obviously the closer thebetter, so that the thickness of the combined surface is minimised. Theminimum spacing is defined by manufacturing requirements and ultimatelyby the wavelength of the radiation involved and the discriminationsensitivity of infrared detectors.

If the arrangement of FIG. 1(c) is used, the secondary reflecting strips18 reflect the energy from the environment below the object. If theenvironment below the object has similar radiance to the background, theheater power required is reduced.

The physical embodiment of the invention uses known reflecting surfacesand known energy transmission surfaces. For example the reflectingsurfaces can comprise tiles or sheets and the radiating strips can bereplaced by appropriate "pin points" for example incandescent filaments,light emitting diodes or the like. However in the preferred embodimentherein described the reflecting strips are conveniently of aluminium orgold suitably coated with a transparent coating, and of the transparentcoatings previously known, by far the most useful is hard carbon ordiamond-like coating which is applied to the surfaces in known manner bymeans of an ion beam generator which directs a beam of energy on to thesurface when the surface is contained within a housing at low pressureand the housing in turn contains a hydrocarbon gas such as methane oracetylene in the presence of hydrogen, this however being a knowntechnique. The coating can be made "selective" that is its opticalproperties can be made to depend on the transmitted wavelength. A hardcarbon or diamond-like coating can contain graphite or other particlesof such size and concentration that it has a low reflectivity in thevisible part of the spectrum but is transparent in the relevantwavelengths generally above 3 micrometers. The films are very hard andable to withstand the rigors of cleaning and general use. The carbon isrefractory, and can alternatively be applied by alternative techniques,including sputtering, evaporation and reactive decomposition.

The radiating strips of metal also coated by hard carbon are firstblackened to increase emissivity thus reducing power requirements forradiation.

The claims defining the invention are as follows:
 1. A means for reflecting and emitting electro-magnetic energy in an appropriate wavelength band comprising an arrangement of surfaces which are reflective to energy in that wavelength band and energy emitters having an emission of energy of such intensity that the combined reflection and emission of said surfaces match energy of a background in that wavelength band.
 2. A means according to claim 1 wherein said reflective surfaces comprise a plurality of surfaces of relatively low emissivity and said energy emitters comprise a plurality of surfaces of relatively high emissivity.
 3. A means according to claim 2 wherein said reflective surfaces comprise some surfaces which are so oriented as to reflect energy from above the horizon.
 4. A means according to claim 2 wherein said reflective surfaces comprise some surfaces which are so oriented as to reflect energy from below the horizon.
 5. A means according to claim 2 wherein said emitters comprise strips of material which, upon energising with an electric current, become heated and radiated energy.
 6. A means according to claim 1 further comprising at least one radiometer, comparison means associated with the radiometer and operative to provide an electrical signal which is a function of the difference between the combined reflection and emission, and of the background, and further comprising a driving circuit controlled by the electrical signal and coupled to the emitters to complete a feedback circuit which varies the energy supplied to the emitters to match the background electro-magnetic energy in the appropriate wavelength band.
 7. A means according to claim 6 wherein there is only one radiometer, and a chopper intercepts the background electro-magnetic energy and the combined reflected and emitter energy of said surfaces.
 8. A means according to claim 6 wherein there is only one radiometer, a chopper rotational about an axis inclined to radiometer, a motor coupled for drive to the chopper, the chopper having an aperture, and a rear reflective surface,further comprising a mirror arranged to reflect said combined reflection and emission to the rear reflective surface of the chopper when the radiometer is directed towards said background, the dimensions of the aperture and rear reflective surface being such that, upon chopper rotation, equal periods of energy of combined reflection and emission, and of energy of background, are sequentially imparted to the radiometer.
 9. A means according to claim 1 further comprising hard carbon coatings on said surfaces.
 10. A means according to claim 1 further comprising diamond-like coatings on said surfaces.
 11. A means according to claim 1 wherein adjacent said low emissivity surfaces are spaced apart by distances not exceeding 100 cm, and adjacent said reflective surfaces are spaced apart by distances not exceeding 100 cm. 